Exhaust gas treatment devices are used on automobiles to reduce atmospheric pollution from engine emissions. Examples of widely used exhaust gas treatment devices include catalytic converters and diesel particulate traps.
A catalytic converter for treating exhaust gases generated an automotive engine includes a housing, a fragile catalyst support structure for holding the catalyst that is used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, and a mounting mat disposed between the outer surface of the fragile catalyst support structure and the inner surface of the housing to hold the fragile catalyst support structure within the housing.
A diesel particulate trap for controlling pollution generated by diesel engines generally includes a housing, a fragile particulate filter or trap for collecting particulate from the diesel engine emissions, and a mounting mat that is disposed between the outer surface of the filter or trap and the inner surface of the housing to hold the fragile filter or trap structure within the housing.
The fragile structure generally comprises a monolithic structure manufactured from a frangible material of metal or a brittle, ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure with a plurality of gas flow channels. These monolithic structures can be so fragile that even small shock loads or stresses are often sufficient to crack or crush them. In order to protect the fragile structure from thermal and mechanical shock and other stresses, as well as to provide thermal insulation and a gas seal, a mounting mat is positioned within the gap between the fragile structure and the housing.
Polycrystalline wool mats may be produced by either a dry laid or wet laid process. Before the drying and calcining stages in the production of polycrystalline wool mats, the sol-gel fibers are flexible. Needling equipment is used at this stage to mechanically interlock the sol-gel fibers while they remain flexible. Following the needling stage, the needled polycrystalline wool mat is dried and calcined. The calcining process renders the sol-gel fibers stiffer.
While the sol-gel fibers remain flexible prior to the drying and calcining stages of the polycrystalline wool mat processing, the sol-gel fibers contain greater than 5 percent water and therefore they are sensitive to exposure to water. Consequently, prior to the drying stage, upon exposure to water used during a wet laid process, the sol-gel fibers would degrade and dissolve. Because of the water sensitivity, only dried and calcined sol-gel fibers are used in a wet laid mat forming process. As only dried and calcined sol-gel fibers are used in the wet laid mat forming process, there is no possibility of needling since any attempt to needle the brittle and stiff sol-gel fibers would result in breaking of the fibers and resulting in a mat with extremely low tensile strength.